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📌 The Question:

In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle?

(a) Carbon cycle
(b) Nitrogen cycle
(c) Phosphorus cycle
(d) Sulphur cycle

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✅ Correct Answer: (c) Phosphorus cycle

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🧠 Classroom Explanation:

• Phosphorus is unique among major biogeochemical cycles because it does not have a significant gaseous phase.
• Most phosphorus on Earth is locked in rocks and sediments as phosphate minerals.

🔹 Primary entry point into the cycle:
👉 Weathering of rocks

• Rain, wind, temperature changes and chemical reactions slowly break down rocks
• This releases phosphate ions (PO₄³⁻) into soil and water
• Plants absorb phosphates → animals get it through food chains
• Eventually, phosphorus returns to sediments and rocks

Hence, weathering of rocks is the dominant natural source, making phosphorus cycle the correct answer.

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🔍 Curiosity Raiser:
🌱 Why is phosphorus often a limiting nutrient in ecosystems?
Because it is released very slowly from rocks, unlike nitrogen or carbon which circulate rapidly through the atmosphere.

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📚 Enrich Notes (Prelims Gold):

Cycle	Main Source of Entry
Carbon	Atmosphere (CO₂)
Nitrogen	Atmosphere (N₂ via fixation)
Phosphorus	Rock weathering
Sulphur	Rocks + atmosphere (volcanoes, SO₂)

🧠 UPSC Trap Insight:
• If the question hints “no gaseous phase” → Think Phosphorus
• If it says weathering is primary source → Again Phosphorus

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🪶 IAS Monk Whisper:
The slowest cycles often decide the fastest collapses.